Lithium disilicate apatite glass-ceramic

ABSTRACT

Lithium disilicate apatite glass-ceramics are described which are characterized by a high chemical stability and can therefore be used in particular as restoration material in dentistry.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser. No. 14/398,599, filed Nov. 3, 2014, which is a National Stage application of International patent application PCT/EP2013/058672 filed on Apr. 25, 2013, which claims priority to European patent application No. 12166760.4 filed on May 4, 2012, the disclosures of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The invention relates to lithium disilicate apatite glass-ceramic which is suitable in particular for use in dentistry, preferably for the preparation of dental restorations, as well as precursors for their preparation.

BACKGROUND OF THE INVENTION

Glass-ceramics with a lithium disilicate and an apatite crystal phase are known from the state of the art.

In Cent. Eur. J. Chem, 7(2), 228-233 (2009), M. Palou et al. report on the crystallization of a mixture of pure lithium disilicate glass and fluoroapatite glass. The glass ceramic produced has a high level, 14 wt.-%, of P₂O₅ and displays bioactivity during in-vitro tests in simulated body fluid.

In Journal of Thermal Analysis and calorimetry 78(1), 73-82 (2004), S. C. Mojumdar et al. describe studies on the crystallization of glasses from the Li₂O—CaO—CaF₂—P₂O₅—SiO₂ system with various levels of P₂O₅. After crystallization of a glass with a 15 wt.-% content of P₂O₅, fluoroapatite was detected in addition to a lithium disilicate crystal phase by means of X-ray diffraction.

However, the lithium silicate glass-ceramics with apatite crystal phase known from the state of the art are bioactive products and not chemically resistant materials which are suitable for restorative dentistry. In body fluids or simulated body fluids, bioactive products form apatite crystals on the surface in order, e.g. in the case of an endoprosthetic implant, to produce a solid bond with the bone.

Therefore, the known glass-ceramics have the serious disadvantage that they do not possess the chemical resistance required for a dental material which comes into contact with a wide variety of fluids in the oral cavity.

SUMMARY OF THE INVENTION

Therefore, the object of the invention is to provide a lithium disilicate apatite glass-ceramic which has a very good chemical resistance and can thus be used as restorative dental material. The glass-ceramic should also be capable of being readily processed into dental restorations, and the restorations produced from it should have very good mechanical and optical properties in addition to a very good chemical stability.

Objects of the invention are achieved by the lithium disilicate apatite glass-ceramic according to the claims. A subject of the invention is also the starting glass according to the claims, the lithium metasilicate glass-ceramic according to the claims, the process according to the claims as well as the use according to the claims.

DESCRIPTION OF THE INVENTION

The lithium disilicate apatite glass-ceramic according to the invention is characterized in that it comprises lithium disilicate as main crystal phase and apatite as further crystal phase and comprises 3.0 to 7.0, in particular 3.5 to 6.0 wt.-% K₂O.

The term “main crystal phase” refers to the crystal phase which has the highest proportion by volume compared with other crystal phases.

Surprisingly, the lithium disilicate apatite glass-ceramic according to the invention is characterized by a very high chemical stability. To determine the chemical stability, the glass ceramic was tested according to ISO standard 6872 (2008) by establishing the mass loss during storage in aqueous acetic acid. The lithium disilicate apatite glass-ceramic according to the invention displayed in particular a mass loss of less than 100 μg/cm², preferably less than 90 and particularly preferably less than 80 μg/cm² and quite particularly preferably less than 50 μg/cm².

The lithium disilicate apatite glass-ceramic preferably comprises 60.0 to 74.0, in particular 63.0 to 71.0 wt.-% SiO₂.

It is also preferred that the lithium disilicate apatite glass ceramic comprises 10.0 to 20.0, in particular 11.0 to 19.0 wt.-% Li₂O.

The molar ratio of SiO₂ to Li₂O is in particular in the range of 1.75 to 3.0.

Furthermore, a lithium disilicate apatite glass-ceramic is preferred which comprises 3.0 to 7.0, in particular 4.0 to 6.0 wt.-% P₂O₅.

In a further preferred embodiment the lithium disilicate apatite glass-ceramic according to the invention comprises 0 to 5.0, in particular 0 to 4.5 wt.-% CaO and 0 to 4.0, in particular 0 to 3.5 wt.-% SrO, wherein the combined amount of CaO and SrO is 1.0 to 6.0, in particular 1.5 to 5.5 wt.-%.

A lithium disilicate apatite glass-ceramic which comprises 0.1 to 1.0, in particular 0.2 to 0.5 wt.-% F is also preferred.

The formation of fluoroapatite is possible by using fluorine. It is particularly preferred that the glass-ceramic according to the invention comprises fluoroapatite as apatite.

In a preferred embodiment the lithium disilicate apatite glass-ceramic also comprises 2.0 to 7.0, in particular 3.0 to 6.0 wt.-% oxide of trivalent elements and/or further oxide of tetravalent elements.

The oxide of trivalent elements is preferably selected from the group of Al₂O₃, Y₂O₃, La₂O₃ and mixtures thereof. Particularly preferably the oxide of trivalent elements is Al₂O₃. Even more preferably the lithium silicate apatite glass-ceramic according to the invention comprises 3.0 to 6.0 wt.-% Al₂O₃.

The term “further oxide of tetravalent elements” refers to oxides of tetravalent elements with the exception of SiO₂. Examples of suitable further oxides of tetravalent elements are ZrO₂, TiO₂ and GeO₂ and mixtures thereof.

Furthermore, a lithium disilicate apatite glass ceramic is preferred which comprises at least one and in particular all of the following components:

Component wt.-% SiO₂ 60.0 to 74.0 Li₂O 10.0 to 20.0 P₂O₅ 3.0 to 7.0, K₂O 3.0 to 7.0 CaO 0 to 5.0 SrO 0 to 4.0 F 0.1 to 1.0 Oxide of trivalent elements and/or further 2.0 to 7.0, oxide of tetravalent elements wherein CaO + SrO is 1.0 to 6.0.

The lithium disilicate apatite glass-ceramic according to the invention can also comprise further additional components which in particular are selected from colorants and fluorescent agents.

Examples of colorants and fluorescent agents are oxides of d- and f-elements, such as the oxides of Ti, V, Sc, Mn, Fe, Co, Ta, W, Ce, Pr, Nd, Tb, Er, Dy, Gd, Eu and Yb. Metal colloids, e.g. of Ag, Au and Pd, can also be used as colorants and in addition can also act as nucleating agents. These metal colloids can be formed e.g. by reduction of corresponding oxides, chlorides or nitrates during the melting and crystallization processes. The metal colloids are preferably present in the glass-ceramic in an amount of from 0.005 to 0.5 wt.-%.

In a preferred embodiment the lithium disilicate apatite glass-ceramic comprises more than 20 vol.-%, preferably more than 25 vol.-% and particularly preferably more than 30 vol.-% lithium disilicate crystals, relative to the total glass-ceramic.

The glass-ceramic according to the invention with lithium disilicate as main crystal phase is characterized by particularly good mechanical properties and it can be formed e.g. by heat treatment of a corresponding starting glass or a corresponding starting glass with nuclei or a corresponding lithium metasilicate glass-ceramic.

It has surprisingly been found out that the lithium disilicate apatite glass-ceramic according to the invention has an excellent chemical stability and also has very good mechanical and optical properties. It is thus superior to the known bioactive lithium silicate apatite glass-ceramics. The combination of its properties even allows it to be used as dental material and in particular material for the preparation of dental restorations.

The lithium disilicate apatite glass ceramic according to the invention has in particular a fracture toughness, measured as K_(IC) value, of at least about 1.6 MPa·m^(0.5) and in particular at least about 1.8 MPa·m^(0.5). This value was determined using the Vickers method and calculated using Niihara's equation. Furthermore, it has a high biaxial breaking strength of preferably from 250 to 550 MPa. The biaxial breaking strength was determined according to ISO 6872 (2008).

The invention also relates to various precursors with the corresponding composition from which the lithium disilicate apatite glass-ceramic according to the invention can be prepared by heat treatment. These precursors are a corresponding starting glass, a corresponding starting glass with nuclei and a corresponding lithium metasilicate glass-ceramic.

The invention therefore also relates to a starting glass which comprises the components of the lithium disilicate apatite glass-ceramic according to the invention.

The starting glass according to the invention therefore also comprises, in addition to 3.0 to 7.0 wt.-% K₂O, in particular suitable amounts of further components required to form the glass-ceramic according to the invention with lithium disilicate as main crystal phase and apatite as further crystal phase. Preferably, it comprises SiO₂ and Li₂O in amounts which make the formation of lithium disilicate possible. Furthermore, the starting glass can also comprises still further components such as are given above for the lithium disilicate apatite glass-ceramic according to the invention. All those embodiments which are also mentioned as preferred for the components of the lithium disilicate apatite glass-ceramic according to the invention are preferred for the components of the starting glass.

The invention also relates to a starting glass which comprises nuclei for the crystallization of lithium metasilicate, lithium disilicate and/or apatite.

Furthermore, the invention relates to a lithium metasilicate glass-ceramic which comprises the components of the lithium disilicate apatite glass-ceramic according to the invention. This lithium metasilicate glass-ceramic therefore also comprises, in addition to 3.0 to 7.0 wt.-% K₂O, in particular suitable amounts of further components required to form the glass-ceramic according to the invention with lithium disilicate as main crystal phase and apatite as further crystal phase. Furthermore, the lithium metasilicate glass-ceramic can also comprise still further components, such as are mentioned above for the lithium disilicate apatite glass-ceramic according to the invention. All those embodiments which are also given as preferred for the components of the lithium disilicate apatite glass-ceramic according to the invention are preferred for the components of the lithium metasilicate glass-ceramic.

By heat treating the starting glass, the further precursors starting glass with nuclei and lithium metasilicate glass-ceramic can firstly be produced. The lithium disilicate apatite glass-ceramic according to the invention can then be produced by heat treating one of these two further precursors. It is preferred to form the lithium disilicate apatite glass-ceramic according to the invention directly by heat treating the starting glass with nuclei.

It is preferred to subject the starting glass to a heat treatment at a temperature of from 430 to 750° C., in particular 430 to 550° C., for a period of 5 to 120 min, in particular 10 to 60 min, in order to produce the starting glass with nuclei for the crystallization of lithium metasilicate, lithium disilicate and/or apatite.

It is further preferred to subject the starting glass with nuclei to a heat treatment at a temperature of more than 600° C. for a period of 5 to 120 min, in particular 10 to 60 min, in order to prepare the lithium metasilicate glass-ceramic or the lithium disilicate apatite glass-ceramic. To prepare the lithium disilicate apatite glass ceramic, the heat treatment of the starting glass with nuclei takes place particularly preferably at 700 to 1000° C., in particular 750 to 900° C., for a period of 5 to 120 min, in particular 10 to 60 min.

The invention also relates to a process for the preparation of the lithium disilicate apatite glass-ceramic according to the invention, wherein the starting glass, the starting glass with nuclei or the lithium metasilicate glass-ceramic is subjected to at least one heat treatment in the range of from 430 to 1000° C.

The at least one heat treatment carried out in the process according to the invention can also take place during a hot pressing or sintering-on of the starting glass according to the invention, of the starting glass according to the invention with nuclei or of the lithium metasilicate glass-ceramic according to the invention.

In a preferred embodiment the process according to the invention comprises

-   -   (a) the heat treatment of the starting glass at a temperature of         from 430 to 550° C. in order to form the starting glass with         nuclei, and     -   (b) the heat treatment of the starting glass with nuclei at a         temperature of from 750 to 950° C. in order to form the lithium         disilicate apatite glass-ceramic.

The duration of the heat treatments carried out in (a) and (b) is in particular 5 to 120 min and preferably 10 to 60 min.

To prepare the starting glass, the procedure is in particular that a mixture of suitable starting materials, such as carbonates, oxides, phosphates and fluorides, is melted at temperatures of in particular from 1300 to 1600° C. for 2 to 10 h. To achieve a particularly high homogeneity, the obtained glass melt is poured into water in order to form a glass granulate, and the obtained granulate is then melted again.

The melt can then be poured into moulds to produce blanks of the starting glass, so-called solid glass blanks or monolithic blanks.

It is also possible to put the melt into water again in order to prepare a granulate. This granulate can be pressed, after grinding and optionally addition of further components, such as colorants and fluorescent agents, to form a blank, a so-called powder green compact.

Finally, the starting glass can also be processed to form a powder after granulation.

The starting glass, e.g. in the form of a solid glass blank, a powder green compact or in the form of a powder, is then subjected to at least one heat treatment. It is preferred that a first heat treatment is initially carried out to prepare a starting glass according to the invention with nuclei which are suitable for forming lithium metasilicate, lithium disilicate and/or apatite crystals. This glass with nuclei is then usually subjected to at least one further temperature treatment at a higher temperature in order to effect crystallization of lithium metasilicate, lithium disilicate and/or apatite.

The further heat treatment for crystallizing lithium metasilicate takes place in particular at a temperature of at least 600° C. For crystallizing lithium disilicate, the further heat treatment takes place in particular at a temperature of at least 700° C. For crystallizing apatite, the further heat treatment takes place in particular at a temperature of at least 800° C.

The glass-ceramics according to the invention and the glasses according to the invention are present in particular in the form of powders, granulates or blanks, e.g. monolithic blanks, such as platelets, cuboids or cylinders, or powder green compacts, in unsintered, partly sintered or densely sintered form. They can easily be further processed in these shapes. They can, however, also be present in the form of dental restorations, such as inlays, onlays, crowns, veneers, shells or abutments.

Dental restorations, such as bridges, inlays, onlays, crowns, veneers, shells or abutments, can be prepared from the glass-ceramics according to the invention and the glasses according to the invention. The invention therefore also relates to their use for the preparation of dental restorations. It is preferred that the glass ceramic or the glass is given, by pressing or machining, the shape of the desired dental restoration.

The pressing usually takes place under increased pressure and increased temperature. It is preferred that the pressing is carried out at a temperature of from 700 to 1200° C. It is further preferred to carry out the pressing at a pressure of from 2 to 10 bar. During pressing, the desired shape change is achieved by viscous flow of the material used. The starting glass according to the invention and in particular the starting glass according to the invention with nuclei, the lithium metasilicate glass-ceramic according to the invention and the lithium disilicate apatite glass-ceramic according to the invention can be used for the pressing. The glasses and glass-ceramics according to the invention can be used in particular in the form of blanks, e.g. solid blanks or powder green compacts, e.g. in unsintered, partly sintered or densely sintered form.

The machining usually takes place by material removal processes and in particular by milling and/or grinding. It is particularly preferred that the machining is carried out in the course of a CAD/CAM process. The starting glass according to the invention, the starting glass according to the invention with nuclei, the lithium metasilicate glass-ceramic according to the invention and the lithium disilicate apatite glass-ceramic according to the invention can be used for the machining. The glasses and glass-ceramics according to the invention can be used in particular in the form of blanks, e.g. solid blanks or powder green compacts, e.g. in unsintered, partly sintered or densely sintered form. Preferably the lithium metasilicate glass-ceramic according to the invention or the lithium disilicate apatite glass-ceramic according to the invention is used for the machining. The lithium disilicate apatite glass-ceramic can also be used in a not yet fully crystallized form which was produced by heat treatment at a lower temperature. This has the advantage that an easier machining and thus the use of simpler equipment for the machining is possible. After the machining of such a partly crystallized material, the latter is usually subjected to a heat treatment at a higher temperature and in particular 700 to 1000° C. and preferably 750° C. to 900° C. in order to effect further crystallization of lithium disilicate and apatite.

In general, after the preparation of the dental restoration shaped as desired, e.g. by pressing or machining, it is in particular heat-treated again in order to convert the precursors used, such as starting glass, starting glass with nuclei or lithium metasilicate glass-ceramic, into lithium disilicate apatite glass-ceramic or to increase the crystallization of lithium disilicate and/or apatite or to reduce the porosity, e.g. of a porous powder green compact used.

However, the glass-ceramics according to the invention and the glasses according to the invention are also suitable as coating material of e.g. ceramics and glass-ceramics. The invention is therefore also directed towards the use of the glasses according to the invention or the glass-ceramics according to the invention for coating in particular ceramics and glass-ceramics.

The invention also relates to a process for coating ceramics and glass-ceramics, wherein glass-ceramics according to the invention or glasses according to the invention are applied to the ceramic or glass-ceramic and are subjected to an increased temperature.

This can take place in particular by sintering-on and preferably by pressing-on. With sintering-on, the glass-ceramic or the glass is applied to the material to be coated, such as ceramic or glass-ceramic, in the usual way, e.g. as powder, and then sintered at increased temperature. With the preferred pressing-on, glass-ceramic according to the invention or glass according to the invention is pressed on, e.g. in the form of powder green compacts or monolithic blanks, at an increased temperature of e.g. from 700 to 1200° C., and by applying pressure, e.g. 2 to 10 bar. The methods described in EP 231 773 and the press furnace disclosed therein can be used in particular for this. A suitable furnace is e.g. the Programat EP 5000 from Ivoclar Vivadent AG, Liechtenstein.

It is preferred that, after conclusion of the coating process, the glass-ceramic according to the invention is present with lithium disilicate as main crystal phase and apatite as further crystal phase, as such a glass-ceramic has particularly good properties.

Because of the above-described properties of the glass-ceramics according to the invention and the glasses according to the invention, these are suitable in particular for use in dentistry. A subject of the invention is therefore also the use of the glass-ceramics according to the invention or the glasses according to the invention as dental material and in particular for the preparation of dental restorations or as a coating material for dental restorations, such as crowns, bridges and abutments.

The invention is explained in more detail below by means of examples.

EXAMPLES Examples 1 to 14 and Comparison Composition and Crystal Phases

A total of 14 glasses and glass-ceramics according to the invention with the composition given in Table I were prepared by melting corresponding starting glasses followed by heat treatment for controlled nucleation and crystallization.

A glass-ceramic not in accordance with the invention which in particular did not contain any K₂O was also produced for comparison.

The heat treatments used for controlled nucleation and controlled crystallization are also given in Table I. The following meanings apply

T_(N) and t_(N) temperature and time used for nucleation T_(k1) and t_(k1) temperature and time used for crystallization of lithium metasilicate T_(k2) and t_(k2) temperature and time used for crystallization of lithium disilicate and apatite

For this, the starting glasses in a range of 100 to 200 g were first melted from customary raw materials at 1400 to 1500° C. The melting was very easily possible without formation of bubbles or streaks. By pouring the starting glasses into water, glass frits were prepared which were then melted a second time at 1450 to 1550° C. for 1 to 3 h for homogenization.

A first heat treatment of the starting glasses at a temperature of from 450 to 470° C. led to the formation of lithium silicate glasses with nuclei. As a result of a further heat treatment at 850° C., these nuclei-containing glasses crystallized to form glass-ceramics with lithium disilicate as main crystal phase and apatite as further crystal phase, as was established by X-ray diffraction tests. Lithium disilicate apatite glass-ceramics were, therefore, obtained.

In the case of Example 4 the heat treatment of the nuclei-containing starting glass at a temperature of merely 700° C. resulted in the crystallization of lithium metasilicate and thus formation of a lithium metasilicate glass-ceramic. This precursor was converted into the corresponding lithium disilicate apatite glass-ceramic by a further heat treatment at 850° C.

The produced lithium disilicate apatite glass-ceramics according to the invention showed an excellent chemical stability according to ISO test 6872 (2008). The mass loss during storage in aqueous acetic acid was less than 100 μg/cm², in particular less than 50 μg/cm².

In contrast to this, the conventional glass-ceramic, prepared for comparison, displayed a very high mass loss of 754 μg/cm² and thus a very low chemical stability. It is not suitable for use as restorative dental material which repeatedly comes into contact with fluids of the most varied composition in the oral cavity.

The lithium disilicate apatite glass-ceramics produced also had high fracture toughness values, measured as critical stress intensity factor K_(IC), of more than 1.8 MPa·m^(0.5).

The biaxial strength σ_(B) was also high, at at least 250 MPa. It was determined according to dental standard ISO 6872 (2008) on test pieces that were prepared by machining of the respective lithium disilicate apatite glass-ceramic. A CEREC®-InLab machine (Sirona, Bensheim) was used for the machining.

The lithium disilicate apatite glass-ceramics produced and the lithium metasilicate glass-ceramic produced as precursor were able to be very satisfactorily machined into the form of various dental restorations in a CAD/CAM process or by hot pressing. These restorations were also provided with a veneer if required.

They were also able to be applied by hot pressing as coatings onto in particular dental restorations, e.g. in order to veneer the latter as desired.

TABLE I Example Comparison 1 2 3 4 5 6 SiO₂ 70.6 69.2 68.8 68.4 68.0 63.9 70.4 P₂O₅ 5.0 5.9 5.9 5.9 5.9 5.9 5.9 Li₂O 17.6 14.4 14.3 14.2 14.1 18.2 11.7 K₂O — 3.8 3.8 3.8 3.8 3.8 3.8 CaO 5.9 2.8 3.3 3.8 4.3 4.3 4.3 SrO — — — — — — — Al₂O₃ — 3.4 3.4 3.4 3.4 3.4 3.4 F 0.9 0.5 0.5 0.5 0.5 0.5 0.5 Mol SiO₂: Li₂O 2.0 2.4 2.4 2.4 2.4 1.75 3.0 T_(g)/° C. 431 443 443 443 442 432 450 T_(N)/° C. 450 460 460 460 460 450 470 t_(N)/min. 10 10 10 10 10 10 10 T_(K1)/° C. — — — — 700 — — t_(K1)/min. — — — — 20 — — T_(K2)/° C. 850 850 850 850 850 850 850 t_(K2)/min. 10 10 10 10 10 10 10 Main crystal phase Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Further crystal phase Ca₅(PO₄)₃F Ca₅(PO₄)₃F Ca₅(PO₄)₃F Ca₅(PO₄)₃F Ca₅(PO₄)₃F Ca₅(PO₄)₃F Ca₅(PO₄)₃F Other crystal phases Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₂SiO₃; Li₃PO₄ quartz, Li₃PO₄ Chemical stability/μg · 754 — — — 6 — — cm⁻² σ_(B)/MPa — — — — 500.6 — — K_(IC)/MPa · m^(0.5) — — — — 2.30 — — Example 7 8 9 10 11 12 13 14 SiO₂ 66.5 66.2 68.7 69.4 68.7 68.7 70.4 68.9 P₂O₅ 5.8 5.8 5.0 4.0 6.0 6.0 5.1 5.0 Li₂O 13.8 13.7 14.3 14.4 14.3 14.3 14.6 14.3 K₂O 5.9 3.7 3.9 3.9 3.9 3.9 4.0 3.9 CaO 4.2 4.2 4.3 4.4 — 1.5 2.0 4.4 SrO — — — — 3.3 1.9 — — Al₂O₃ 3.3 5.9 3.4 3.4 3.4 3.4 3.5 3.4 F 0.5 0.5 0.5 0.5 0.5 0.4 0.5 0.2 Mol SiO₂: Li₂O 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.4 T_(g)/° C. 444 444 441 443 443 435 442 450 T_(N)/° C. 460 460 460 460 460 450 460 470 t_(N)/min. 10 10 10 10 10 10 10 10 T_(K1)/° C. — — — — — — — — t_(K1)/min. — — — — — — — — T_(K2)/° C. 850 850 850 850 850 850 850 850 t_(K2)/min. 10 10 10 10 10 10 10 10 Main crystal phase Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Further crystal phase Ca₅(PO₄)₃F Ca₅(PO₄)₃F Ca₅(PO₄)₃F Ca₅(PO₄)₃F Sr₅(PO₄)₃F Sr_(7.3)Ca_(2.7)(PO₄)₆F₂ Ca₅(PO₄)₃F Ca₅(PO₄)₃F Other crystal phase Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₂SiO₃, Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₃PO₄ Chemical stability/μg · — — — 13.0 — 13.0 — 11.0 cm⁻² σ_(B)/MPa — — — 500 — 401 — 285 K_(IC)/MPa · m^(0.5) — — — — — — — —

Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims. 

1. Lithium disilicate apatite glass-ceramic, which comprises lithium disilicate as main crystal phase and apatite as further crystal phase and which comprises 3.0 to 7.0 wt.-% K₂O.
 2. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 60.0 to 74.0 wt.-% SiO₂.
 3. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 10.0 to 20.0 wt.-% Li₂O.
 4. Lithium disilicate apatite glass-ceramic according to claim 1, which contains 3.0 to 7.0 wt.-% P₂O₅.
 5. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 0 to 5.0 wt.-% CaO and 0 to 4.0 wt.-% SrO, wherein the combined amount of CaO and SrO is 1.5 to 5.5 wt.-%.
 6. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 0.1 to 1.0 wt.-% F.
 7. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 2.0 to 7.0 wt.-% oxide of trivalent elements and/or further oxide of tetravalent elements.
 8. Lithium disilicate apatite glass-ceramic according to claim 7, wherein the oxide of trivalent elements is selected from the group of Al₂O₃, Y₂O₃, La₂O₃ and mixtures thereof.
 9. Lithium disilicate apatite glass-ceramic according to claim 7, wherein the further oxide of tetravalent elements is selected from the group of ZrO₂, TiO₂, GeO₂ and mixtures thereof.
 10. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises at least one of the following components: Component wt.-% SiO₂ 60.0 to 74.0 Li₂O 10.0 to 20.0 P₂O₅ 3.0 to 7.0 K₂O 3.0 to 7.0 CaO 0 to 5.0 SrO 0 to 4.0 F 0.1 to 1.0 Oxide of trivalent elements and/or further 2.0 to 7.0 oxide of tetravalent elements wherein CaO + SrO is 1.5 to 5.5.


11. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises fluoroapatite as apatite.
 12. Lithium disilicate apatite glass ceramic which comprises lithium disilicate as main crystal phase and apatite as further crystal phase, which comprises 3.0 to 7.0 wt.-% K₂O, and which comprises more than 20 vol.-% lithium disilicate crystals.
 13. Starting glass, which comprises the components of the glass-ceramic according to claim
 1. 14. Starting glass according to claim 13, which comprises nuclei for the crystallization of lithium metasilicate, lithium disilicate and/or apatite.
 15. Lithium metasilicate glass-ceramic, which comprises lithium metasilicate as main crystal phase and apatite as further crystal phase and which comprises 3.0 to 7.0 wt.-% K₂O.
 16. Glass-ceramic according to claim 1, wherein the glass-ceramic is present in the form of a powder, a granulate, a blank or a dental restoration.
 17. Process for the preparation of the glass-ceramic according to claim 1, wherein a starting glass or a lithium metasilicate glass-ceramic is subjected to at least one heat treatment in the range of from 430° to 1000° C.
 18. Process according to claim 17, wherein (a) the starting glass is subjected to a heat treatment at a temperature of from 430 to 550° C. in order to form starting glass with nuclei, and (b) the starting glass with nuclei is subjected to a heat treatment at a temperature of from 750 to 950° C. in order to form the lithium disilicate apatite glass-ceramic.
 19. Process of using the glass-ceramic according to claim 1, as dental material comprising coating dental restorations and preparing dental restorations.
 20. Process of using the glass-ceramic according to claim 19, wherein the process comprises pressing or machining, the shape of the desired dental restoration, bridge, inlay, onlay, veneer, abutment, partial crown, crown or shell.
 21. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 3.5 to 6.0 wt.-% K₂O.
 22. Lithium disilicate apatite glass-ceramic according to claim 2, which comprises 63.0 to 71.0 wt.-% SiO₂.
 23. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 11.0 to 19.0 wt.-% Li₂O.
 24. Lithium disilicate apatite glass-ceramic according to claim 1, which contains 4.0 to 6.0 wt.-% P₂O₅.
 25. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 0 to 4.5 wt.-% CaO and 0 to 3.5 wt.-% SrO, wherein the combined amount of CaO and SrO is 1.5 to 5.5 wt.-%.
 26. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 0.2 to 0.5 wt.-% F.
 27. Lithium disilicate apatite glass-ceramic according to claim 1, which comprises 3.0 to 6.0 wt.-% oxide of trivalent elements and/or further oxide of tetravalent elements.
 28. Lithium disilicate apatite glass-ceramic according to claim 7, wherein the oxide of trivalent elements is selected from the group of Al₂O₃, Y₂O₃, La₂O₃ and mixtures thereof.
 29. Lithium disilicate apatite glass ceramic according to claim 1, which comprises more than 25 vol.-% lithium disilicate crystals.
 30. Lithium disilicate apatite glass ceramic according to claim 1, which comprises more than 30 vol.-% lithium disilicate crystals.
 31. Starting glass according to claim 13, wherein the starting glass is present in the form of a powder, a granulate, a blank or a dental restoration.
 32. Lithium metasilicate glass-ceramic according to claim 15, wherein the lithium metasilicate glass-ceramic is present in the form of a powder, a granulate, a blank or a dental restoration.
 33. Process of using the starting glass according to claim 13 as dental material comprising coating dental restorations and preparing dental restorations.
 34. Process of using the lithium metasilicate glass-ceramic according to claim 15 as dental material comprising coating dental restorations and preparing of dental restorations. 